Heavy duty motor vehicle transmissions are commonly configured to accept a Power-Take-Off (PTO) unit for operating various accessory devices such as winches and dump lifts. In a typical mechanization, the transmission includes a PTO drive gear coupled to the transmission input shaft, and the PTO drive gear is maintained in meshing engagement with an input gear of the PTO unit. The PTO input gear drives a PTO input shaft, and a dedicated hydraulic clutch in the PTO unit selectively couples the PTO input shaft to a PTO output shaft for engagement and disengagement of the PTO. However, this approach adds complexity and cost to the PTO unit, and tends to increase gear noise even when the PTO is not activated. Accordingly, what is needed is an improved way of selectively activating and deactivating the PTO function.